1. Field of the Disclosure
The present disclosure relates generally to optical fibers and optical fiber lasers and amplifiers. The present disclosure relates more particularly to mode mixing optical fibers useful, for example in providing optical fiber laser and amplifier outputs having a desired beam product parameter and beam profile, as well as in the construction of laser and amplifier systems.
2. Technical Background
High power lasers and optical amplifiers are widely used in a variety of industries for a variety of purposes, such as laser cutting, welding and machining of various materials. Research and development in rare-earth doped optical fibers along with the discovery of specialty fiber designs such as Large-Mode Area (LMA) fibers has triggered the introduction of a variety of high power fiber laser and amplifier modules. Multi-kW fiber lasers and amplifiers have been realized with very high efficiencies and are fueling the growth of laser material processing. Of course, other types of high power lasers, such as solid-state lasers, are also commonly used in materials processing applications.
Lasers and amplifiers used in the field of materials processing desirably fulfill specific requirements in terms of output power and beam profile. In terms of power, the laser or amplifier system desirably delivers radiation with a wavelength and an energy that is high enough to process a desired material, typically on the order of kilowatts. Two sorts of kW-level fiber lasers can be distinguished: multi-mode and single-mode. Single-mode fiber lasers typically deliver on the order of 1-3 kW of optical power, while multi-mode fiber lasers typically operate in the range of several tens of kW of output power. For material processing applications, both single mode and multi-mode fiber lasers are used. A multi-mode laser can be configured, for example, by using a multi-mode active fiber, or by combining the outputs of several single mode fiber lasers into a multi-mode delivery fiber for delivery to a workpiece. Similarly, a multi-mode delivery fiber is often used to deliver power from a solid-state laser to a workpiece.
In terms of beam profile, users typically desire the delivered beam to have a desired Beam Parameter Product (BPP). As used herein, the BPP is defined as the product of the beam radius R and the divergence angle of the beam θ, expressed in units of mm·mrad. The beam radius R in mm is half of the Beam Diameter measured at 13.5% of the maximum intensity. The divergence angle θ in mrad is defined as the half-angle formed with the optical axis as the beam propagates from the end of a beam delivery optical fiber. While desired BPP values will vary from application to application, three typical ranges of BPP values for fiber-coupled lasers are provided below:                1.5 to 2 mm·mrad for a 50 μm core diameter beam delivery cable        3 to 4 mm·mrad for a 100 μm core diameter beam delivery cable        6 to 8 mm·mrad for a 200 μm core diameter beam delivery cable        
Moreover, in many applications, the delivered beam has an intensity profile that is substantially evenly distributed along the beam. Such a “flat-top” profile is different from a Gaussian profile, in which the maximum intensity is only at the center. A “flat-top” profile can help to enable controlled and accurate cutting, welding or machining process.
In many applications, a beam with a substantially circular profile is also (or alternatively) desired.
In order to use such lasers for material processing applications while satisfying the required beam parameter product (BPP), conventional optical fiber laser and amplifier systems have a single mode or multi-mode laser or amplifier output coupled into a beam delivery cable for transmission of the output to a workpiece. Similarly, conventional solid-state lasers are coupled to a beam delivery cable for transmission of the laser output to a workpiece. Commonly used beam delivery cables are made with highly multi-mode step-index fibers with typical core diameters of 50, 100, 200, 400 and 600 microns and numerical apertures (NA) varying from 0.1 to 0.4 (and often greater than 0.4). A number of techniques have been attempted to provide both a desired BPP and a desired flat-top profile, such as offset splicing between a single mode laser output (launch fiber) and the beam delivery cable, beam delivery optical fibers with shaped cores, external beam shaping techniques, mechanical fiber micro-bending, fiber tapers (adiabatic and/or abrupt), long period gratings and multimode interference in multi-mode fibers. However, each of these suffers from a number of drawbacks.
Accordingly, there remains a need for improved optical fibers, systems and methods that can, for example, provide one or more of a desired BPP value, a desired intensity profile (e.g., a “flat top” intensity profile), and a circular beam shape.